Photovoltaic power plant

ABSTRACT

In large PV power plants, grounding of individual PV modules may lead to problems. The present invention overcomes such problems. The basis for the invention is a PV power plant comprising one or more PV generators, each comprising a PV string and an inverter with a DC input and an AC output. The PV string comprises at least one PV module and is electrically connected to the DC input of the inverter. The inverter comprises means for controlling the DC potential at the DC input depending on the DC potential at the AC output. The AC outputs of the inverters are coupled in parallel. The novel feature of the invention is that the PV power plant further comprises an offset voltage source, which controls the DC potential at the AC outputs. Thereby, the DC potential at the DC input will be indirectly controlled, and it is thus possible to ensure that the potentials with respect to ground at the terminals of the PV modules are all non-negative or all non-positive without grounding the PV modules. Ground loops can be avoided, and there is no need for the use of transformer-based inverters.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a reissue of U.S. application Ser. No. 13/127,813that issued as U.S. Pat. No. 9,287,712 that issued on Mar. 15 2016 thatis entitled to the benefit of and incorporates by reference essentialsubject matter disclosed in International Patent Application No.PCT/DK2009/000231 filed on Nov. 6, 2009 and Danish Patent ApplicationNo. PA 2008 01537 filed Nov. 7, 2008; and Danish Patent Application No.2009 00896 filed on Jul. 24, 2009.

FIELD OF THE INVENTION

The invention regards a photovoltaic (PV) power plant.

BACKGROUND OF THE INVENTION

In larger PV power plants with DC/AC-converters (inverters), the plantis typically connected to the power grid through a dedicated isolationtransformer, which connects the relatively low voltage PV generatorsystem to the medium voltage power grid. One reason for this is that thePV modules, which convert the solar energy into electrical energy,typically must have a defined potential with respect to ground. This istypically achieved by grounding all or some of the PV modules.

Grounding is normally done in order to comply with local regulations, tofacilitate the detection of isolation faults and/or to avoid corrosionand/or yield reduction of the PV modules.

Detection of isolation faults may be difficult in larger systems due tothe rather high leakage currents from the PV modules, especially in wetconditions. By grounding the system, leakage currents can be monitored.

Some types of PV module, notably thin film modules incorporating a TCO(transparent conductive oxide) layer are prone to irreparable damage,and consequent substantial power losses, resulting from the reaction ofglass-sodium with moisture. To avoid accelerated degradation of such PVmodules, it is normally required to ground the negative terminal of thePV strings, i.e. avoid that any active part of the PV modules have anegative potential with respect to the ground potential. The degradationof the PV modules depends on the potential difference between the activeparts of the module and the ground. Depending on the moduleconstruction, grounded parts may be in very close distance from theactive parts—accelerating the degradation.

With some other types of PV module, notably those where the terminalsare all located on one side of the module—known as ‘back contactmodules’—a reduction of module efficiency has been observed duringoperation. This appears to be due to a buildup of static charge on thesurface of the cell and can be counteracted by maintaining the cellbelow the ground potential. Thus, some back-contact PV modules requirethat the positive terminal is grounded in order to avoid yield losses,i.e. their terminals must have non-positive potentials.

In larger PV systems comprising several strings of PV modules andseveral inverters, grounding of more than one PV module may causecurrents to run through the ground (ground loops). Ground loops maycause problems with controlling the power plant, increase the riskand/or the rate of corrosion and also increase problems relating toelectromagnetic interference (EMI). In order to avoid ground loops,transformer-based inverters may be used, so that the DC and the AC sidesof the inverters are separated galvanically. Such inverters are however,relatively heavy and expensive, and there is a demand for PV powerplants, which may utilise transformer-less inverters and still ensuredefined potentials with respect to ground at the PV modules. The use oftransformer-less inverters in large PV power plants does however,require that, if ground loops shall be avoided, the PV generator systembe configured as a network with an earthing system where the AC side ofthe inverters has no connection to ground at all. This is known as an‘IT’ earthing system and is described in, for example, IEC(International Electrotechnical Commission) International Standard60364-1—Electrical Installation in Buildings. This means, in practice,that the AC side of the system must be floating with respect to groundand can therefore not be grounded.

FIG. 1 illustrates a typical prior art power plant 22 and comprises asingle PV generator 23, comprising a PV string 3 and a transformerless(non galvanically isolated) inverter 24. The inverter 24 has a DC input18 and a three-phase AC output 19. The PV string 3 comprises three PVmodules 5 connected in series and arranged so that they will be exposedto sunlight. Each PV module 5 comprises a number of PV cells (not shown)connected as already known in the art so that they generate a single DCpower output at the terminals 6 of the PV module 5. The PV string 3 iselectrically connected to the DC input 18 of the inverter 4 through apositive connection 7 and a negative connection 8. The AC outputs 19 areconnected electrically in parallel to a power grid 9 comprising threepower lines and a neutral line. The neutral line is connected to groundvia a ground connection 15.

Here the system is configured as a network with an earthing system wherethe AC side of the inverter has a connection to ground, and the networkalso includes a ground connection. This is known as a ‘TN’ earthingsystem and is described in, for example, IEC 60364-1.

When operating, the voltages appearing at the positive input 7 andnegative input 8 of the inverter 24 are represented in FIG. 2 . In thegraph the axis 25 represents the voltage with respect to ground, and itwill be seen that the voltage at the positive input 7 (represented bythe line 27) is above ground potential whilst that at the negative thenegative input 8 (represented by the line 28) is below ground potential.These voltages, as well as the potential between them—the voltage acrossthe PV string 3 (represented by the range 26)—are controlled by thecharacteristics of the inverter 24, the irradiation of the PV string 3,the type of solar cells used in each PV module 5 as well as otherfactors. Since the grounding of either side of the inverter DC inputs 18is not possible in this design of power plant, such a power plant 22will be subject to a decrease in efficiency, and the PV string 3 liableto damage, resulting from the problems discussed above.

FIG. 3 illustrates another prior art power plant 29. Here the differencefrom the power plant 22 of FIG. 1 is that the PV generator 31 comprisesa transformer-based (galvanically isolated) inverter 30, that is to saythere is galvanic isolation between the DC input 18 and AC output 19 ofthe inverter. This allows the grounding of the negative input 8 of theinverter 30 to be made using a ground connection 32. FIG. 4 illustratesthe voltages appearing at the DC inputs 18 of the inverter 30 in asimilar manner to FIG. 2 . In can be seen that the whole PV string 3 isheld at a positive potential relative to ground. Such a configuration issuitable for avoiding the problems with thin film modules discussedabove. If, alternatively, the positive input 7 of the inverter 30 wasgrounded instead of the negative input 8, then a configuration suitablefor back contact type modules would be realised.

In this type of power plant, whilst it is possible to control thevoltages appearing at the inputs 18, and so minimise the decrease inefficiency and damage resulting from the problems discussed above, thisadvantage comes only at the cost of using a transformer-based inverter(30). Such an inverter design is more expensive to produce, heavier andis less efficient in operation and so the use of such an inverter isclearly a disadvantage. A further disadvantage of this type of powerplant is the requirement that a ground connection 32 needs to bephysically connected to the positive input 7 or the negative input 8 ofthe inverter 30. This requires additional hardware and labour to attach.In addition, changing the type of PV modules at a later date may involvethe physical disconnection and/or reconnection of a ground connection32, a procedure which is labour intensive and therefore a disadvantage.

FIG. 5 illustrates yet another prior art power plant 33. Here thedifference from the power plant 22 of FIG. 1 is that the AC outputs 19are connected electrically in parallel to a power grid 9 through athree-phase AC connection 17 and a three-phase isolation transformer 10having a primary side 11, a secondary side 12 and a neutral terminal 13on the primary side. Such a transformer is often used in high capacitypower plants, where multiple inverters are coupled in parallel, and isdescribed in more detail below. Such a configuration allows thepotential of the inverter inputs 18 to be independent of the potentialof the network 9 without the need for using a costly transformer-basedinverter 31. The grounding of the negative input 8 of the inverter 24can be made using a ground connection 32. FIG. 4 illustrates thevoltages appearing at the DC inputs 18 of the inverter 24 in a similarmanner to FIG. 2 . In can be seen that the whole PV string 3 is held ata positive potential relative to ground. Such a configuration issuitable for avoiding the problems with thin film modules discussedabove. If, alternatively, the positive input 7 of the inverter 24 wasgrounded instead of the negative input 8, then a configuration suitablefor back contact type modules would be realised.

A disadvantage of this type of power plant is the requirement that aground connection 32 needs to be physically connected to the positiveinput 7 or the negative input 8 of the inverter 24. This requiresadditional hardware and labour to attach. In addition, changing the typeof PV modules at a later date may involve the physical disconnectionand/or reconnection of a ground connection 32, a procedure which islabour intensive and therefore a disadvantage. In addition, if two ormore PV generators 23 are connected in parallel, as illustrated in FIG.6 , and one input of each inverter 23 is earthed as described above,problems will arise if the characteristics or irradiation of each PVstring 3 are not identical. This may cause unwanted voltages andconsequent ground loop currents.

In all the prior art power plants illustrated above it can be seen thatwhilst the potential of one or more PV strings with respect to ground isan important parameter for running a power plant in an efficient mannerand one which does not cause damage to the PV strings, this often onlyachieved by the use of expensive hardware or the labour intensivefitting of additional hardware.

SUMMARY OF THE INVENTION

Therefore, it is an object of the invention to provide a PV power plantcomprising PV strings in which the grounding regime of the PV stringshinders degradation and loss of efficiency.

It is a further object of the invention to provide a PV power plantwhich easy to adapt to different types of PV string.

It is an even further object of the invention to provide a PV powerplant in which the grounding regime of the PV strings is programmableand thus continuously adaptable. The aim of the present invention is toovercome the above mentioned and other drawbacks of known PV powerplants.

The basis for the invention in a first aspect is a PV power plantcomprising a PV generator, the PV generator comprising a PV string andan inverter with a DC input and an AC output. The PV string comprises atleast one PV module and is electrically connected to the DC input of theinverter. The novel feature of the invention is that the PV power plantfurther comprises an offset voltage source, which controls the DCpotential at the AC outputs. Thereby, the DC potential at the DC inputwill be indirectly controlled, and it is thus possible to ensure thatthe potentials with respect to ground at the terminals of the PV modulesare all non-negative or all non-positive without grounding the PVmodules. Thus, ground loops can be avoided, and there is no need for theuse of transformer-based inverters. Furthermore, isolation faults in thepower plant may be detected by monitoring the current flowing from theoffset voltage source.

Preferably, the inverter comprises means for controlling the DCpotential at the DC input depending on the DC potential at the ACoutput. Such means may be dedicated to this function or the function maybe a by-product of another function within the inverter.

The PV power plant may also preferably comprise one or more additionalPV generators where the AC outputs of all the inverters of these PVgenerators are coupled in parallel.

Preferably, the output voltage of the offset voltage source depends onthe solar irradiation on and/or the ambient temperature of the PVmodules. This allows for controlling the potentials at the terminals ofthe PV modules so that they are as close to ground potential as possibleat all times.

The output voltage of the offset voltage source may additionally oralternatively depend on an external reference voltage. This allows forcontrolling the potentials at the terminals of the PV modules accordingto any preset potential, or a potential that is set remotely and/ordynamically in order to compensate for factors not immediatelyaccessible to the PV power plant or its components.

The output voltage of the offset voltage source may additionally oralternatively depend on the measured potential of one or more of theinputs to one or more of the inverters. This allows for controlling thepotentials at the terminals of the PV modules according to the potentialacross them produced by irradiation or, alternatively or additionally,according to a requirement to hold one or more of the inverters withincertain limits, for example within a certain potential relative toground.

The output voltage of the offset voltage source may additionally oralternatively is time dependent. This allows for controlling thepotentials at the terminals of the PV modules for example according tothe time of day. This is an advantage, for example, if a reversepotential is required during the hours of darkness to repair damagecaused to ‘back contact modules’ during daylight hours, or for varyingthe potential of the PV string throughout the day following the patternof expected irradiation. Additionally or alternatively a time dependencymight follow a yearly or weekly cycle dependent upon a preset patternconnected with ambient temperature or power usage.

In one embodiment of the invention, the offset voltage source maypreferably comprise at least one offset PV module. A PV moduleconstitutes a very reliable voltage source, is easy to incorporate intoa PV power plant and eliminates the need for installing any additionalpower generators.

The offset PV modules may preferably be are arranged so that they willbe subjected to the same solar irradiation and/or the same ambienttemperature as the PV modules. This is a very simple way of achievingthat the potentials at the terminals of the PV modules can be as closeto ground potential as possible at all times.

Preferably, the output voltage of the offset voltage source may equalapproximately half of the output voltage of the PV strings, and theinverters may comprise an electrical equalising circuit, which causesthe DC potential at their DC inputs to be symmetric around the averageDC potential at their AC outputs. This establishes an even simplercontrol of the PV module potentials.

The power plant may preferably further comprise an isolation transformerhaving a primary side connected to the AC outputs, a secondary side anda neutral terminal on the primary side, and the offset voltage sourcemay be connected between ground and the neutral terminal. This is a verysimple way of controlling the DC potential at the AC outputs of theinverters.

The AC outputs of the inverters and the isolation transformer maypreferably comprise three phases. In this way, a very stable neutralterminal may be achieved.

Alternatively or additionally the offset voltage source may form part ofone of the inverter. This would allow for a compact and efficientsystem. This may preferentially be realised by one inverter becoming a‘controlling’ inverter which supplies the DC offset to all the inverterson the isolated AC side of the isolation transformer.

Alternatively or additionally the offset voltage source may beprogrammable. By this is meant that the offset voltage source can beprogrammed, by means of inbuilt computer code or other means, to respondin a simple or complex manner to inputs such as temperatures, voltagemeasurements, time, power usage requirements or other parameters.

Alternatively or additionally the offset voltage source may be able tobe turned off. By this is meant that the offset voltage source can betaken out of circuit. This has the distinct advantage that when theoffset voltage source is not required (for example during night-timehours when no power is being produced) the power that it uses may besaved resulting in a more efficient PV power plant.

The basis for the invention in a second aspect is realised by a methodof controlling a PV power plant, the PV power plant comprising at leastone inverter with a DC input electrically connected to a PV string, anAC output and a means for controlling the DC potential at the DC inputdepending on the DC potential at the AC output, the method comprisingthat of controlling the DC potential at the AC outputs by use of anoffset voltage source.

This aspect of the invention may advantageously be realised by the stepof adjusting the voltage of the voltage source to hold the voltage ofone of the DC inputs at a voltage offset with respect to ground.

In one embodiment of the method the DC input may be the positiveconnection, in another embodiment of the method the DC input may be thenegative connection.

Alternatively or additionally this aspect of the invention mayadvantageously be realised by making the voltage offset substantiallyzero.

Alternatively or additionally this aspect of the invention mayadvantageously be realised by further the step of turning the offsetvoltage source off. This is advantageous for the reasons describedpreviously.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention and its advantages will become more apparent, whenlooking at the following description of possible embodiments of theinvention, which will be described with reference to the accompanyingfigures, which are showing:

FIG. 1 shows a schematic diagram of a prior art PV power plant,

FIG. 2 shows a graph illustrating voltages obtained in the prior art PVpower plant shown in FIG. 1 ,

FIG. 3 shows a schematic diagram of another prior art PV power plant,

FIG. 4 shows a graph illustrating voltages obtained in the prior art PVpower plants shown in FIGS. 3 and 5 ,

FIG. 5 shows a schematic diagram of yet another prior art PV powerplant,

FIG. 6 shows a schematic diagram of a modified form of the prior art PVpower plant shown in FIG. 5 ,

FIG. 7 shows a first embodiment of a PV power plant according to theinvention,

FIG. 8 shows a graph illustrating voltages obtained in the embodiment ofthe PV power plant shown in FIG. 7 ,

FIG. 9 shows a second embodiment of a PV power plant according to theinvention,

FIG. 10 shows a third embodiment of a PV power plant according to theinvention, and

FIG. 11 shows a schematic of an isolation transformer shown in FIGS. 5,7, 9 and 10 .

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The PV power plant 34 of FIG. 7 comprises a PV generator 2, comprising aPV string 3 and an inverter 4. The inverter 4 has a DC input 18 and athree-phase AC output 19. The inverter 4 comprises an electricalequalising circuit, which causes the DC potential at its DC input 18 tobe symmetric around the average DC potential at its AC output 19. In itssimplest form, the equalisation circuit may comprise a voltage dividerbased on resistors, inductors and/or capacitors. The PV string 3comprises three PV modules 5 connected in series and arranged so thatthey will be exposed to sunlight. Each PV module 5 comprises a number ofPV cells (not shown) connected as already known in the art so that theygenerate a single DC power output at the terminals 6 of the PV module 5.The PV string 3 is electrically connected to the DC input 18 of theinverter 4 through a positive connection 7 and a negative connection 8.The AC outputs 19 are connected electrically in parallel to a power grid9 through a three-phase AC connection 17 and a three-phase isolationtransformer 10 having a primary side 11, a secondary side 12 and aneutral terminal 13 on the primary side. An offset voltage source 14 iselectrically connected between ground 15 and the neutral terminal 13.

The PV power plant 34 functions as follows. The PV modules 5 convert theradiation energy received from the sun into electrical energy andthereby generate DC voltages across their terminals 6. Due to the seriesconnection of the PV modules 5, a PV string DC voltage appears betweenthe positive connection 7 and the negative connection 8. In typical PVpower plants, the PV string DC voltages may be as high as above 1,000 V.The inverter 4 converts the PV string DC voltage at its DC input 18 intoa three-phase AC voltage at its AC output 19, from where it is led tothe power grid 9 through the AC connection 17 and the isolationtransformer 10. The inverter 4 is controlled by a control system (notshown) to ensure that no electrical power flows from the AC output 19 tothe DC input 18. The power plant 34 thus converts solar energy intoelectrical energy, which is delivered to the power grid 9. The PV stringDC voltages and thus the output power of the power plant 34 vary withthe irradiation and the ambient temperature as is already known in theart.

The offset voltage is applied to the neutral terminal 13 of theisolation transformer 10, thereby causing the average DC potential atits primary side 11 to be offset from ground potential with the offsetvoltage. Thus, also the average DC potential with respect to ground 15at the AC output 19 of the inverter 4 equals the offset voltage. Due tothe equalising circuit in the inverter 4, the potentials with respect toground 15 at the positive connection 7 and the negative connection 8will be symmetrical around the offset voltage, i.e. approximately zeroat one of the connections 7, 8 and approximately twice the offsetvoltage at the other connection 7, 8. By selecting an appropriateelectrical polarity for the offset voltage source 14, it can thus beensured that the potentials with respect to ground for all PV modules 5are, for example, either non-negative or non-positive and nearly alwaysvery close to ground potential.

When operating, the voltages appearing at the positive input 7 andnegative input 8 of the inverter 4 are represented in FIG. 8 . In thisfigure the axis 25 represents the voltage with respect to ground, and itwill be seen that the voltage at the positive input 7 (represented bythe line 27) is above ground potential whilst that at the negative input8 (represented by the line 28) is below ground potential. Thesevoltages, as well as the potential between them—the voltage across thePV string 3 (represented by the range 26)—are controlled by thecharacteristics of the inverter 4, the irradiation of the PV string 3,the type of solar cells used in each PV module 5 as well as otherfactors. The arrows 35 and 36 illustrate that fact that the potentialsat the positive 7 and negative 8 inputs of the inverter 4 can be variedby the variation of the voltage output by the offset voltage source 14.

The isolation transformer 10 shown in FIG. 11 comprises threestar-coupled primary windings 20 on the primary side 11 and threestar-coupled secondary windings 21 on the secondary side 12. The centralconnection point of the primary windings 20 constitutes the neutralterminal 13 of the isolation transformer 10.

In the case that the PV modules 5 are of the ‘thin film’ type, theoffset voltage source can be driven so that the whole PV string 3 isheld at a positive potential relative to ground. Such a configuration issuitable for avoiding the problems with thin film modules discussedabove. If, alternatively, the offset voltage source 14 is driven so thatthe positive input 7 of the inverter 4 is kept at or near groundpotential, then a configuration suitable for back contact type modulesis realised. The advantages of this embodiment are clear to see: sincethere is no requirement for the inverter 4 to be of a transformer-based(galvanically isolated) type, cost and weight can be reduced andefficiency improved.

Turning now to FIG. 9 we see a second embodiment of a PV power plantaccording to the invention. This PV power plant 37 is similar to the PVpower plant 34 of the first embodiment above, but with the addition oftwo or more PV generators 39, similar in design to the PV generator 2 ofthe first embodiment. Again, each PV generator 39 comprises a PV string3 (not shown) and an inverter 38. Each inverter 38 has a DC input 18 anda three-phase AC output 19. The AC outputs 19 are connected electricallyin parallel to a power grid 9 through a three-phase AC connection 17 anda three-phase isolation transformer 10. As before, an offset voltagesource 14 is electrically connected between ground 15 and the neutralterminal 13.

FIG. 9 also illustrates a controller 40 which controls the voltage andpolarity of the offset voltage source 14 though a control line 41. Thesignal on the control line 41 is a function of the output of acomparator 42 which compares the output of a voltage measurement 45, 46with respect to ground 15 and a reference voltage 43. The voltagemeasurement can be either the voltage of the positive input 7 or thenegative input 8 of the inverter 4. The choice of this voltage is madeby means of a switch 47. The switch 47 may be a physical switch (forexample controlled directly by service personnel), or an electronicswitch.

It would, of course, be possible to build the functionality of thecontroller 40, comparator 42 and switch 47 into the inverter 4. In thiscase, inverter 4 becomes a ‘controller’ inverter which supplies the DCoffset to all the inverters on the isolated AC side of the isolationtransformer 10.

The advantages of this embodiment are similar to the advantages alreadygiven for the first embodiment discussed above. In addition, it will beseen that there is no requirement to ground the appropriate input ofeach inverter 4, 38 individually since the offset voltage source 14controls the voltage relative to ground on the isolated AC side of allthe inverters 4, 38 to a reference point. This reference point could beset to any desired potential between positive or negative side of the PVstring and thus compensate for different problems associated withdifferent PV cell type discussed above.

The reference point could also be made programmable, that is it can bevaried according to the type of PV string being used, or by some othercriteria. It also could be set as a function of time and thus it wouldbe possible to changed the settings of the offset voltage during the dayif required.

Since the offset voltage is being produced at a single point in thecircuit, and simultaneously alters the potential to ground of all the PVmodules 5, there are no voltage differences between the PV modules 5,and no related ground loops between the inverters 4, 38.

Since very little current flows through the voltage source, there isvery little power dissipated (often of the order of 1 Watt).

FIG. 10 shows a third embodiment of the invention. This is similar tothe embodiment illustrated in FIG. 7 , but with the addition of a secondPV generator 2, and with each PV string 3 comprising four PV modules 5connected in series. The offset voltage source 14 comprises two offsetPV modules 16 connected in series. The offset PV modules 16 are similarin construction to the PV modules 5 of the PV generators 2.

The number of offset PV modules 16 equals half the number of PV modules5 in a PV string 3, wherefore the output voltage of the offset voltagesource 14—the offset voltage—equals approximately half of the PV stringDC voltages. Most of the time, the offset voltage source 14 is lessloaded than the PV strings, wherefore most of the time, the offsetvoltage will be a little higher than half of the PV string DC voltages.

Instead of using an equalising circuit, the DC potential at the DC input18 of the inverters 4 may be controlled actively by the inverter controlcircuits. This is for instance possible in a transformer-less inverterwith an unsymmetrical boost circuit.

Although various embodiments of the present invention have beendescribed and shown, the invention is not restricted thereto, but mayalso be embodied in other ways within the scope of the subject-matterdefined in the following claims.

What is claimed is:
 1. A photovoltaic (PV) power plant comprising a PVgenerator, the PV generator comprising a PV string and an inverter witha DC input and an AC output, the PV string comprising at least one PVmodule and being electrically connected to the DC input, wherein the PVpower plant further comprises an offset voltage source, which controlsDC potential at the AC outputs, the offset voltage source beingconnected to an AC side of the inverter between ground and a neutralterminal of the AC output.
 2. The PV power plant according to claim 1,in which the inverter comprises a means for controlling the DC potentialat the DC input depending on the DC potential at the AC output.
 3. Thephotovoltaic (PV) power plant according to claim 1, further comprisingone or more additional PV generators each comprising a PV string and aninverter with a DC input and an AC output, the AC outputs output of eachof the inverters being coupled in parallel.
 4. The photovoltaic (PV)power plant according to claim 1, in which the an output voltage of theoffset voltage source depends on one or more of: the a solar irradiationof the PV modules, the an ambient temperature of the PV modules, anexternal reference voltage and the a measured potential of one or moreof the at the DC inputs input of the inverter.
 5. The photovoltaic (PV)power plant according to claim 1, in which the an output voltage of theoffset voltage source is time dependent.
 6. The photovoltaic (PV) powerplant according to claim 1, in which the offset voltage source comprisesat least one offset PV module.
 7. The photovoltaic (PV) power plantaccording to claim 6, in which the at least one offset PV modules moduleis are arranged so that they it will be subjected to the same solarirradiation and/or the same ambient temperature as the at least one PVmodules module.
 8. The photovoltaic (PV) power plant according to claim1, in which the an output voltage of the offset voltage source equalsapproximately half of the output voltage of the PV strings string, andwherein the inverter comprises an electrical equalising circuit, whichcauses the DC potential at its DC input to be symmetric around theaverage DC potential at its AC output.
 9. The photovoltaic (PV) powerplant according to claim 1, in which the power plant further comprisesan isolation transformer having a primary side connected to the ACoutputs output, a secondary side and a neutral terminal on the primaryside, and that the offset voltage source is connected between ground andthe neutral terminal.
 10. The photovoltaic (PV) power plant according toclaim 9, in which the AC outputs output and the isolation transformercomprise one or more phases.
 11. The photovoltaic (PV) power plantaccording to claim 1, in which the offset voltage source forms part ofone inverter.
 12. The photovoltaic (PV) power plant according to claim1, in which the offset voltage source is programmable and/or can beturned off.
 13. A method of controlling a PV power plant, the PV powerplant comprising at least one inverter with a DC input electricallyconnected to a PV string, an AC output and a means for controlling theDC potential at the DC input depending on the DC potential at the ACoutput, an offset voltage source associated with the AC output, themethod comprising that of controlling the a DC potential at the ACoutputs output by use of an the offset voltage source connected to an ACside of the at least one inverter between ground and a neutral terminalof the AC output.
 14. The method of claim 13 further comprising the stepof adjusting the offset voltage of the voltage source to hold thevoltage of one the DC inputs input at a voltage offset with respect toground.
 15. The method of claim 14 in which the voltage offset issubstantially zero.
 16. The method according to claim 13, furthercomprising the step of turning the offset voltage source off.
 17. Thephotovoltaic (PV) power plant of claim 1, wherein the inverter comprisesa three-phase AC output.
 18. The photovoltaic (PV) power plant of claim1, wherein the neutral terminal comprises a central connection point ofthree star-coupled windings.
 19. The photovoltaic (PV) power plant ofclaim 1, wherein the neutral terminal is connected to the inverter. 20.A PV power plant comprising a PV generator, the PV generator comprisinga PV string and an inverter with a DC input and an AC output, the PVstring comprising at least one PV module and being electricallyconnected to the DC input, wherein the PV power plant further comprisesan offset voltage source, which controls DC potential at the AC output,the offset voltage source being connected to an AC side of the inverterbetween ground and a neutral terminal of circuitry connected to the ACoutput.
 21. A system, comprising: an inverter configured to couple to aPV string comprising at least o ne PV module at a DC input thereof, andfurther configured to couple to an AC grid at an AC output thereof; andan offset voltage source configured to control DC potential at the ACoutput, wherein the offset voltage source is configured to be connectedat an AC side of the inverter between ground and a neutral terminalassociated with the AC output.
 22. The system of claim 21, wherein theneutral terminal comprises a central connection point of threestar-coupled windings.
 23. The system of claim 21, wherein the neutralterminal is connected to the inverter.